# Cumulative Incidence vs. Incidence Rate: What’s the Difference?

Two terms that are commonly used by statisticians in the healthcare and epidemiology industries are incidence rate and cumulative incidence.

These terms provide us with slightly different information and are both useful to know when studying a disease.

The incidence rate refers to the rate that a disease occurs in a population relative to the total observation time that individuals are at risk.

It is calculated as:

• Incidence rate = # New cases of disease during observation period / Total observation time at risk

The cumulative incidence refers to the portion of people in a population who develop a disease during a specific observation time period relative to the total number of people at risk at the beginning of an observation time period.

It is calculated as:

• Cumulative incidence = # people at risk who develop the disease during observation period / Total people at risk at beginning of observation time period

The following example shows how to calculate both of these metrics in practice.

## Example: How to Calculate Incidence Rate & Cumulative Incidence

Suppose a statistician is studying a particular disease in a population. For simplicity, we’ll refer to it as disease A.

Suppose the statistician wants to calculate both incidence rate and cumulative incidence to understand the rate that this disease occurs in a population along with the portion of individuals who can be expected to catch the disease.

Assume that statistician completes a study with the following data:

• Time period: 1 year
• Number of individuals at beginning of study: 10,000
• Number of new cases of disease during study: 400

The statistician can use the following formula to calculate the incidence rate:

• Incidence rate = # New cases of disease during observation period / Total observation time at risk
• Incidence rate = 400 / 10,000 * 1 year
• Incidence rate = 400 / 10,000 person-years

We would say that the incidence rate is 400 new cases per 10,000 person-years.

If we’d like, we can also divided by 10,000 to express this number in terms of 1 person-years.

For example, this is the same as .04 new cases per 1 person-years.

The statistician can use the following formula to calculate the cumulative incidence:

• Cumulative incidence = # people at risk who develop the disease during observation period / Total people at risk at beginning of observation time period
• Cumulative incidence = 400 / 10,000
• Cumulative incidence = .04

We would say that the cumulative incidence of this particular disease is .04. In simple terms, this helps us understand that 4% of the total population at risk caught the disease during the 1-year time frame of the study.

## When to Use Cumulative Incidence vs. Incidence Rate

In most studies, it’s useful to calculate both the cumulative incidence and the incidence rate since both metrics provide us with different information.

The cumulative incidence gives us an idea of the portion of individuals who catch a disease, which will always range between 0% and 100%.

Conversely, the incidence rate gives us an idea of how often the disease occurs using person-years as the denominator.

By using both of these metrics, a statistician can summarize the rate at which a particular disease occurs.